Solid state electrochemical devices offer advantages over conventional sealed wet cell devices, such as, for example, nickel cadmium, nickel metal-hydride, alkaline, and lead-acid batteries. These conventional devices, and numerous others, are produced in sealed packages such as metal cans, or rigid plastic containers. This is primarily because of their dependence on fluid electrolyte, and the need to contain the electrolyte. Containers of this type add considerable weight to the device assembly, and limit the form the device may take.
Electrochemical devices, including batteries and electrochemical capacitors, however, do not necessarily require liquid electrolyte, and therefore do not necessarily require rigid packaging to act as an exoskeleton. A common requirement for an electrochemical device package, however, is that it be hermetically sealed to prevent gases from entering the package. Furthermore, it is desirable to have a flexible package for solid state electrochemical devices if the solid state cells themselves are flexible, thus providing a flexible energy source. This reduces the design constraints for host devices, such as, for example, cellular phones, which could use solid state energy sources.
Accordingly, a variety of flexible packaging structures have been developed for electrochemical devices. One example can be found in U.S. Pat. No. 5,057,385 to Hope et al. Briefly, the preferred package material discussed therein is a laminar structure comprising a metal foil layer sandwiched between polymeric layers, formed into an envelope which is heat sealed. The metal foil layer is used only as barrier to moisture and gasses. This laminate provides not only a flexible package, but also one which is much lighter than conventional rigid packages. This is considered a significant market advantage in weight sensitive portable products, such as cellular phone markets.
One of the chief considerations in designing a package for a flat electrochemical device is how to provide electrical connections from the cell or cell stack inside the package to electrical contacts outside the package while maintaining the hermetic integrity of the package. In the design discussed in the above referenced patent, a conductor tab is extended from the cell inside the package between the two envelope sides to the outside, bonding the laminate to the tab. In practice this is a very difficult seal to maintain. Plastic to metal bonds of this type tend to be weak and short lived, particularly if the package is flexed often, or subject to mechanical shock. Since the problem with this design is partly due to the length of the bonds, a subsequent design sought to improve on this by using conductor mesh, or screen material instead of tab stock. This allowed the polymeric layers to bond to each other between the wires of the mesh, thus significantly reducing the path length of the plastic to metal bonds. However, although this proved to be an improvement, it still was not robust enough for use with many portable electrical devices.
A more recent approach to this problem can be found in U.S. Pat. Nos. 5,456,000 and 5,460,904. These patents are commonly assigned, and describe similar subject matter. The problem of the plastic to metal seal was eliminated by the use of rivets and O-ring seals. This is a similar approach used in conventional canned cells, and has proven to be very robust in those devices. A tab is extended from the current collector, and the rivet passes through the tab and the package. The package is an envelope comprising sides formed by a pair of polymeric layers separated by a metal foil layer, similar to that used to package foodstuffs, and similar to the laminate used by Hope, above. The rivet compresses a coaxial O-ring located inside the package with enough force to maintain an airtight seal. This approach works well, but adds significant cost to the package, both in material and assembly. In addition, the rivet provides opportunity for failure. For Example, the O-ring may be flawed, or become torn during assembly, or may not be placed at all. Therefore it would be desirable to find a less complicated and less expensive method of providing external connections.
A rivet-less approach has been designed and implemented by Gould Electronics Inc. on their Powerdex.RTM. line of planar primary lithium batteries. The packaging is essentially an adhesively backed polymeric label with a pair of openings formed therethrough. The packaging laminate has no metal foil layer. The cell structure comprises two copper current collectors, one for each electrode. One collector is directly exposed through one of the openings in the label, and the other is extended from the cell, and folded over to the same side of the cell as the first collector. The two collectors are separated by a layer of card stock paper board, through which a hole is formed corresponding with one of the openings to expose the first collector. The second collector has a portion corresponding with the second opening in the label, and is exposed through the second opening. This approach works well for primary cells, but lacks the robust hermeticity required for rechargeable electrochemical devices used with portable electronic devices. Further, the label assembly method, while a significant improvement over the rivet method, still leaves much to be desired for high volume manufacturing.
A simpler method than the Gould method is disclosed in U.S. Pat. No. 4,623,598. The package disclosed therein provides both contacts on the same side of the package, and it is easily sealable to provide the level of hermeticity necessary for portable products. The package is formed from a sheet which has two halves. A collector is disposed on each half. However, in order to provide both contacts on the same side of the package, the first collector extends from its half onto the other half. This necessarily reduces the area available for the second collector. Further, in order to insulate the portion first collector which extends onto the second half from the active material of the second collector contacts, an insulating layer is required between the active material and the extended portion. This can further reduce the area of active material which the second collector may come in contact with. In order to provide a contact large enough to be useful, a significant portion of the active material will not be accessible to the second collector, and therefore will not be utilized. This has the effect of reducing the capacity of the electrochemical cell.
Therefore, there exists a need for a flat electrochemical device package which provides a robust hermetic seal, is easily assembled, allows full utilization of the active materials, and provides a cost advantage over known packaging methods.